A fuel cell converts chemical energy directly to electrical energy by supplying a fuel and an oxidant to two electrically-connected electrodes and causing electrochemical oxidation of the fuel. Unlike thermal power generation, fuel cells are not limited by Carnot cycle, so that they can show high energy conversion efficiency. In general, a fuel cell is formed by stacking a plurality of single fuel cells each of which has a membrane electrode assembly as a fundamental structure, in which an electrolyte membrane is sandwiched between a pair of electrodes. Especially, a solid polymer electrolyte fuel cell which uses a solid polymer electrolyte membrane as the electrolyte membrane is attracting attention as a portable and mobile power source because it has such advantages that it can be downsized easily, operated at low temperature, etc.
In a solid polymer electrolyte fuel cell, the reaction represented by the following formula (A) proceeds at an anode electrode (fuel electrode) in the case of using hydrogen as fuel:H2→2H++2e−  Formula (A)
Electrons generated by the reaction represented by the formula (A) pass through an external circuit, work by an external load, and then reach a cathode electrode (oxidant electrode). Protons generated by the reaction represented by the formula (A) are, in the state of being hydrated and by electro-osmosis, transferred from the anode electrode side to the cathode electrode side through the solid polymer electrolyte membrane.
In the case of using oxygen as an oxidant, the reaction represented by the following formula (B) proceeds at the cathode electrode:2H++(½)O2+2e−→H2O  Formula (B)
Water produced at the cathode electrode passes through a gas channel and so on and is discharged to the outside. Accordingly, fuel cells are clean power source that produces no emissions except water.
In a solid polymer electrolyte fuel cell, the electricity generation performance is largely affected by the amount of water in the electrolyte membrane and electrodes. In particular, if the water (emission) is excessive, the water condensed inside the fuel cell fills a void in the electrodes and, further, the gas channels to interrupt the supply of reaction gases (fuel gas and oxidant gas), so that the reaction gases for electricity generation are not sufficiently distributed throughout the electrodes. As a result, there is a problem that there is an increase in concentration overvoltage and thus a decrease in power output and electricity generation efficiency of the fuel cell. On the other hand, if the water inside the fuel cell is insufficient and thus the electrolyte membrane and electrodes are dried, there is a decrease in proton (H+) conductivity of the electrolyte membrane and electrodes. As a result, there is a problem that there is an increase in resistance overvoltage and thus a decrease in power output and electricity generation efficiency of the fuel cell.
Also in the solid polymer electrolyte fuel cell, a non-uniform distribution of water occurs in a plane direction of the electrolyte membrane (that is, a plane direction of the electrodes), which means that water is unevenly distributed in the plane direction of the electrolyte membrane. As a result, a non-uniform distribution of electricity generation occurs in the plane direction of the electrolyte membrane, resulting in a further uneven distribution of water and thus a decrease in power output and electricity generation efficiency of the fuel cell.
As described above, to realize a solid polymer electrolyte fuel cell with high power output and high electricity generation efficiency, appropriate water control is very important. In order to avoid water shortage, especially so-called drying up (dry-up), it is proposed to supply humidified reaction gases. In this case, however, the above problems due to excessive water are more likely to occur. In addition, as a result of equipping the fuel cell with a humidifier, the fuel cell becomes larger and the fuel cell system becomes complex, for example.
Therefore, there has been an attempt to obtain stable electricity generation performance by appropriately controlling the moisture state of the fuel cell under a non-humidified condition in which the reaction gases are not humidified.
For example, Patent Literature 1 discloses a fuel cell system which is operated under a non-humidified condition and/or high temperature condition and which prevents in-plane moisture distribution of a fuel cell from occurring by determining the dry state near the inlet of an oxidizing agent gas channel based on the resistance of the fuel cell, the voltage of the fuel cell, or the pressure loss of the oxidizing agent gas, and then controlling the flow rate or pressure of the fuel gas based on the determination.
As a technique for controlling the moisture state in the fuel cell, for example, Patent Literature 2 discloses a fuel cell system which comprises a current sensor for measuring an output current value of the fuel cell, a voltage sensor for measuring an output voltage value of the fuel cell, and a storage means for memorizing the relationship between the output voltage value and output current value, the relationship being the basis for determining whether the operation state of the fuel cell is an optimum operating state or not, which retrieves an optimum voltage value corresponding to the measured current value measured by the current sensor from the storage means, and which determines that the moisture state of the fuel cell is a dry state when the difference between the retrieved optimum voltage value and the measured voltage value measured by the voltage sensor is larger than the preset threshold value.
Patent Literature 3 discloses a fuel cell system which comprises a measuring device for measuring voltage in a plurality of measuring points of the fuel cell, and which estimates the uneven distribution of water in the fuel cell based on the difference of moisture contents between the plurality of measuring points, which were estimated from the difference of the voltage values measured in different measuring points.
Patent Literature 4 discloses a fuel cell system which determines whether the execution condition for performing the moisture content state determination of the fuel cell is filled or not from the time sequential change of voltage of the fuel cell based on the drop width of voltage corresponding to transitional load increase, and which determines the moisture state of the fuel cell when the execution condition is determined to be filled, based on the drop width of the voltage and the time sequential change of the electric resistance of the fuel cell.